The invention relates to electric appliances in general, and more particularly to improvements in electric appliances which consume water while in use. Typical examples of such appliances are coffee, espresso and capuccino making machines and tea making machines. For the sake of simplicity and convenience, the invention will be described with reference to electric coffee making machines; however, it is to be understood that the invention can be embodied with similar or equal advantage in many other types of water-consuming appliances for the making of hot beverages.
An electric coffee maker normally comprises a housing which is provided with a vessel for a supply of tap water. An electric heater is installed in the housing to heat a stream of water which flows from the vessel into a receptacle for comminuted coffee beans. The thus obtained beverage can be dispensed into a coffee can or another suitable receptacle which can rest on a warming plate. As a rule, the vessel is filled with tap water which normally contains various gases as well as salts drawn from the soil or from rocks with which the water comes in contact prior to entering a reservoir serving as a source of water for the taps in a dwelling or in another establishment. Salts which are customarily found in tap water include sodium bicarbonate, sodium sulfate and magnesium bicarbonate. The percentage of sodium and magnesium salts determines the hardness of water which is normally measured in degrees, such as the English or Clark degrees, the German degrees and the French degrees. The number of degrees is higher if the water is harder, and vice versa.
Hardness of water which is used in an electric coffee making machine exerts a great influence upon the useful life of such appliance. The reason is that heating entails a decomposition of water into its constituents, and these constituents (especially carbonates) deposit on the adjacent surfaces in the form of scale. The scale is an insulator of heat so that the energy requirements of the machine increase as the thickness of scale increases. In addition, scale gradually clogs various pipes, conduits and other components which define relatively narrow paths for the flow of water in the interior of a coffee making machine so that the rate of water flow becomes highly unpredictable. In fact, scale can completely clog the conduits so that the machine must be discarded even though many or all other component parts are still ready for use.
Heretofore known proposals to eliminate scale include the utilization of various acids. Thus, if the scale gathers on parts which are made of copper, brass or certain other metals, it can be removed with diluted lactic or acetic acid. The makers recommend periodic descaling of such appliances; the length of intervals between successive descaling operations depends upon the hardness of water which is used in the coffee making machines.
It is further known to employ so-called calcification or hardness indicators in the form of mechanical, electric or electronic instruments which contribute significantly to complexity and cost of the appliances. Moreover, presently known instruments of such character are not overly reliable which is another reason why they failed to gain widespread acceptance in the relevant industry. Reference may be had, for example, to German Offenlegungsschrift No. 32 23 969 which discloses an indicator serving to inform the user of a coffee making machine that a decalcification of conduits is advisable or necessary. The operation of the indicator which is disclosed in this published German patent application is based on the recognition that the transfer of heat between the electric heater and water which flows from the vessel for tap water to the receptacle for comminuted coffee beans is less satisfactory when the internal surface of the conduit serving to convey water through the heater is coated with boiler scale. Thus, when the internal surface of such conduit is coated with scale, the temperature of the water heater rises and the rise of temperature is proportional to the quantities of scale in the conduit. A discrete (additional) thermostat is provided to monitor the temperature of the water heater and to initiate the generation of a signal, e.g., by causing a lamp to light up, when the temperature of the water heater rises to a preselected value which is indicative of excessive deposition of scale in the conduit. The lamp thus informs the person in charge that a decalcification is in order.
A drawback of the just described indicators is that they are not reliable. For example, an overheating of the water heater can also take place when the supply of water in the conduit which is to convey water through the heater is evaporated in its entirety. This can induce the operator to carry out a decalcifying or scale-removing operation before such operation becomes necessary. Therefore, the published German patent application further proposes that the calcification indicator be combined with an electronic clock having a memory to store information pertaining to the interval of time which normally elapses between the instant of turning on the electric heater and first disconnection of the heating element from its energy source by the customary thermostat of the machine. The clock is designed to initiate the generation of a signal which denotes that a decalcification of the conduit or conduits in the machine is necessary if the temperature of the heating element rises beyond an acceptable value prior to elapse of the aforementioned interval. Thus, the clock serves as a means for taking into account that the interval of time which elapses for complete evaporation of a given quantity of water is longer if the conduits contain substantial quantities of scale than if the conduits are free to permit water to flow therein at the desired or expected rate. This is due to the aforediscussed fact that the exchange of heat between the heating device and a stream of water which flows therethrough is less satisfactory if the internal surface or surfaces of the conduit or conduits which convey water through the heater are coated with layers of scale. In other words, the interval of time which is required for complete evaporation of a given quantity of tap water is prolonged proportionally with the quantity of scale which coats the internal surface or surfaces of the conduit or conduits leading from the vessel for a supply of tap water to the receptacle for comminuted coffee beans. The provision of an electronic clock contributes to reliability of the indicator; however, such clock also contributes significantly to the initial and maintenance cost of the appliance because it is expensive and complex and hence prone to malfunction. The total number of parts in such indicators and in the associated clocks is large, and their space requirements are excessive.